This invention relates to a frequency characteristic compensation circuit such as an equalizer amplifier which has an RC circuit network.
Equalizer amplifiers are classified generally into two types: RC (resistance-capacitance) attenuation type as shown in FIGS. 1A and 1B, and NFB (negative feedback) type as shown in FIG. 2.
In the known equalizer amplifier shown in FIG. 1A, an output signal of an amplifier 1 is applied to an RC circuit 2 having predetermined equalizer characteristics for amplitude correction. An output signal of the RC circuit is amplified by another amplifier 3 and led to an output terminal 4. Explanation will be made below of the case where the RC circuit 2 has known RIAA reproduction characteristics in which signal attenuation increases with increase in signal frequency. In this case, the RC circuit 2 is comprised of a resistor 5 of a resistance value R1 connected between the output of the amplifier 1 and the input of the amplifier 3, a capacitor 6 of a capacitance value C1 and a resistor 7 of a resistance value R2 connected in series between the input of the amplifier 3 and circuit ground, and a capacitor 8 of a capacitance value C2 connected in parallel with the resistor 7.
The RC circuit 2 has a transfer function G1 which is represented as follows: ##EQU1## where s=j.omega.. Transfer function Gr for the RIAA reproduction characteristics is expressed by the following formula: ##EQU2## where time constant T1=3180 .mu.sec, T2=318 .mu.sec, and T3=75 .mu.sec.
As is evident from formulas (1) and (2), formula (1) can be rendered identical with formula (2) if R1, R2, C1 and C2 are properly selected.
In the known equalizer amplifier shown in FIG. 1B, and RC circuit 9 is comprised of a resistor 10 of a resistance value R3 connected between the output of amplifier 1 and the input of amplifier 3, a capacitor 11 of a capacitance value C3 and a resistor 12 of a resistance value R4 connected in series between the input of amplifier 3 and circuit ground, and a capacitor 13 of a capacitance value C4 connected in parallel with the capacitor 11 and the resistor 12. The RC circuit 9 has a transfer function G2 which is represented as follows: ##EQU3##
Comparision of formula (3) with formula (2) shows that the RC circuit 9 illustrated in FIG. 1B can have the RIAA reproduction characteristics.
Both equalizer amplifiers mentioned above of RC attenuation type can provide accurate RIAA reproduction characteristics. But they are disadvantageous with respect to signal to noise (S/N) ratio and distortion characteristics. To improve the S/N ratio it is necessary that the output impedance of the RC circuit should be low since this impedance serves as an input resistance of the amplifier 3. To lower the output impedance of the RC circuit it is necessary to use capacitors of a large capacitance. From a viewpoint of electrical characteristics, however, a capacitor of a small capacitance is superior to a capacitor of a large capacitance.
In the equalizer amplifier of NFB type shown in FIG. 2, 2, an RC negative feedback circuit comprised of resistors 18 and 19 and capacitors 16 and 17 is connected between the input and output of an amplifier 15. With this circuit the feedback amount or factor increases with increase in the frequency of an input signal. As a result, the gain of amplifier 15 of FIG. 2 decreases with increasing frequency of the input signal. The equalizer amplifier has a transfer function G3 which is represented as follows: ##EQU4## where, R5 and R6 denote the resistance values of the resistors 18 and 19, respectively, Re the resistance value of an input resistor 20 of the amplifiers 15, and C5 and C6 the capacitance values of the capacitors 16 and 17, respectively. Being of negative feedback type, the equalizer amplifier of FIG. 2 is advantageous over the equalizer amplifiers of FIGS. 1A and 1B with respect to S/N ratio and distortion characteristics. But it is relatively difficult for the equalizer amplifier of FIG. 2 to obtain accurate RIAA reproduction characteristics, as will be evident from formula (4).